Effect of Helmet in Preventing Human Head and Brain Trauma ...Many studies have been conducted to...

Effect of Helmet in Preventing Human Head and Brain Trauma in Traffic Accidents

Farshad Tavallalinia

Submitted to the Institute of Graduate Studies and Research

in partial fulfillment of the requirements for the Degree of

Master of Science in

Mechanical Engineering

Eastern Mediterranean University June 2013

Gazimağusa, North Cyprus

Approval of the Institute of Graduate Studies and Research

Prof. Dr. Elvan Yılmaz

Director I certify that this thesis satisfies the requirements as a thesis for the degree of Master of Science in Mechanical Engineering. Assoc. Prof. Dr. Uğur Atikol Chair, Department of Mechanical Engineering We certify that we have read this thesis and that in our opinion it is fully adequate in scope and quality as a thesis for the degree of Master of Science in Mechanical Engineering.

Assist. Prof. Dr. Neriman Özada Supervisor

Examining Committee

1. Assist. Prof. Dr. Hasan Hacisevki

2. Assist. Prof. Dr. Mostafa Ranjbar

3. Assist. Prof. Dr. Neriman Özada

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ABSTRACT

To date, there have been number of studies to demonstrate the effectiveness of helmet

during head impacts in car or motorcycle accidents, but these studies were focused on

frontal impacts to the head only. There is still a need to study more on helmet to make it

more effective, especially in lateral impacts. To my knowledge, previous studies did not

focus on the effects of helmet during lateral impacts. This study focuses on lateral

impacts and how the helmet prevents head injuries during such impacts. In this thesis,

3D models of the human head and helmet were created in order to build a parameterized

Finite Element (FE) model of a protected human head. In the FE model, brain is

assumed to be hyper-elastic, the helmet is considered as elasto-plastic and the skull is

considered as a linear elastic material.

Two separate FE analyses were carried out in this study to find out how the helmet

prevents lateral impacts to the head. In the first analysis, the head was not protected and

it hits the impact object directly. In the second analysis the head model is protected with

a one-layered helmet. In each of these simulations normal and Von Mises stresses are

calculated and in the end compared with each other in order to understand the influence

of helmet in reduction of such stresses on human head. Afterwards the acceleration of

center mass of the head during two simulations are calculated and compared to each

other. Helmet reduces head acceleration by 83%, which is a very important cause of

Traumatic Brain Injury (TBI). It is clear that the helmet can be very crucial in avoiding

brain trauma during car accidents under lateral impacts.

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Through applying more sophisticated modeling in the FEA, the stress distribution at

each layer of the helmet and at each part of the head could be illustrated. Therefore, this

knowledge will provide insight into designing and developing helmet and safety

products to reduce head and brain trauma.

Keywords: Traumatic Injury, Helmet, Lateral Impacts, Head, Biomechanics

v

ÖZ

Bu güne kadar yapılan bir çok araştırmada, araba ve motor kazalarında kask

kullanımının etkisi incelenmiş fakat bu araştırmalar, kazalarda başın ön tarafına gelen

darbeleri ele almıştır. Pek çok trafik kazası sonucunda baş yaralanmaları yan taraftan

alınan darbelerden dolayı da olabilir. Fakat hala daha yan taraftan alınan darbelerin kask

ile nasıl zararsız hale getirileceği incelenmemiştir. Yaptığım araştırmalara dayanarak,

çok az bilimsel çalışmada başın aldığı yan darbeler ele alınmıştır. Bu araştırma tezi,

başın aldığı yan darbelerde kask kullanımını ve yaralanmalardaki etkilerini incelemiştir.

Bu tezde ilk olarak yapılan, üç boyutlu baş ve kask modellerinin yaratılmasıdır. Daha

sonra bu modeler sonlu elemanlar yazılımına aktarılmış ve analize hazır duruma

getirilmiştir. Sonlu elemanlarda kullanılan modellerde, kafatası doğrusal ve elastik,

beyin, hiper elastik ve kask elasto-plastik olarak tanımlanmıştır.

Sonlu elemanlar kullanılarak iki tür analiz yapılmıştır. Bunlardan birincisinde kask

kullanılmamış ve başa alınan yan darbelerde, başın gösterdiği mekanik davranış elde

edilmiştir. Ikinci çalışmada, hem baş hemde kask modelleri kullanılmıştır. Her iki

analizde de hem normal hemde Von Mises stress dağılımları elde edilmiş ve birbirleriyle

karşılaştırılmıştır. Burada elde edilen sonuçlardan kask kullanımının başa gelen yan

darbenin şiddetini düşürdüğü ve başı koruduğu anlaşılmıştır. Daha sonra ivme

hesaplamaları iki tür analiz için de yapılmıştır. Böylece yan darbelerde başı korumak

için kask kullanımının önemi ortaya konulmuştur.

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Bu çalışma ileride daha sofistike kask ve baş modellerinin yaratılmasının analizlerde

daha detaylı sonuçlar elde edilmesinde yararlı olacağını desteklemektedir. Bunların

yanında elde edilen detaylı analiz sonuçları da beyin ve baş travmalarını önlemeye

yarayan kask ve benzeri koruyucu ürünlerin tasarım ve geliştirilmesine katkıda

bulunacaktır.

Anahtar Kelimeler: Travmatik Yaralanma, Kask, Yan Darbeler, Baş, Biyomekanik

vii

ACKNOWLEDGMENTS

I would like to express my deepest appreciation to my supervisor, Assist. Prof. Dr.

Neriman Ozada for her invaluable help and support during the last 2 years that I was

working on the project and for being there whenever I needed help.

I also would like to thank my parents and my brother for their invaluable support and

encouragement during my studies and I know that I would not have been here without

them.

viii

TABLE OF CONTENTS

ABSTRACT ..................................................................................................................... iii

ÖZ ...................................................................................................................................... v

ACKNOWLEDGMENTS ............................................................................................... vii

LIST OF FIGURES .......................................................................................................... xi

LIST OF ABBREVIATIONS ......................................................................................... xiv

1 INTRODUCTION .......................................................................................................... 1

1.1 Need for Head and Brain Modeling in Traffic Accident Analysis .......................... 1

1.2 Anatomy of the Human Head .................................................................................. 3

1.2.1 Scalp .................................................................................................................. 3

1.2.2 Skull .................................................................................................................. 4

1.2.3 Membrane ......................................................................................................... 4

1.2.4 Cerebrospinal Fluid (CSF) ................................................................................ 4

1.2.5 Brain .................................................................................................................. 4

1.2.6 Cerebral Vasculature ......................................................................................... 5

1.3 Head and Brain Trauma in Car/Motorcycle Accidents ............................................ 5

1.3.1 Skull Fracture .................................................................................................... 5

1.3.2 Epidural Hematoma .......................................................................................... 6

1.3.3 Subdural Hematoma .......................................................................................... 6

1.3.4 Cerebral Contusion ........................................................................................... 6

1.3.5 Diffuse Axonal Injury ....................................................................................... 7

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Preventing Head and Brain Trauma by Using Helmet .................................................. 7

1.4.1 Helmet shell and liner ....................................................................................... 8

1.4.2 Fiberglass versus plastic shells ......................................................................... 8

Organization of the Thesis ............................................................................................. 8

2 LITERATURE REVIEW ............................................................................................. 10

2.1 Head Injury Prediction ........................................................................................... 10

2.2 Development of Head, Neck and Torso Models for Simulating Car/Motorcycle

Crashes ......................................................................................................................... 11

2.3 Modeling and Analysis of Head Trauma During Traffic Accidents ...................... 15

3 MATERIALS AND METHODS .................................................................................. 19

3.1 Construction of 3D Models .................................................................................... 19

3.1.1 3D Model of the Helmet ................................................................................. 19

3.1.2 Brain and Head ............................................................................................... 21

3.1.3 Final Assembly ............................................................................................... 22

3.2 Developing a Finite Element Model of Head and Brain ........................................ 23

3.2.1 Meshing ........................................................................................................... 23

3.2.3 Material properties .......................................................................................... 26

3.2.4 Contacts between brain and skull ................................................................... 29

3.3 Finite Element Model of Head and Brain with Helmet ......................................... 30

3.4 Impact conditions and Finite Element analysis of the model ................................ 32

4 RESULTS ..................................................................................................................... 34

4.1 Normal Stress Analysis .......................................................................................... 35

4.1.1 Helmet ............................................................................................................. 35

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4.1.2 Skull ................................................................................................................ 37

4.1.3 Brain ................................................................................................................ 41

4.2 Von Mises Equivalent Stress ................................................................................. 45

4.2.1 Helmet ............................................................................................................. 45

4.2.2 Skull ................................................................................................................ 46

4.2.3 Brain ................................................................................................................ 47

4.3 Acceleration on Center of mass of the head .......................................................... 49

5 DISCUSSION ............................................................................................................... 50

5.1 Discussion of the Results ....................................................................................... 51

5.2 Key Research Accomplishments ........................................................................... 54

6 CONCLUSION AND FUTURE WORK ..................................................................... 56

REFERENCES ................................................................................................................ 58

APPENDIX ...................................................................................................................... 67

Appendix 1 ................................................................................................................... 68

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LIST OF FIGURES

Figure 3.1: A Half Sphere Created as a Part of the Helmet Model ................................. 19

Figure 3.2: A Boss Extrude with 15 Degrees Angle is Created from the Center of the

Sphere ...................................................................................................................... 20!

Figure 3.3: The Helmet Model is Converted Into a Shell ................................................ 21!

Figure 3.4: Final Geometry of the Helmet Model After a Surface Cut is Performed. ..... 21!

Figure 3.5: Assembly of the Skull and Brain Models ...................................................... 22!

Figure 3.6: Final Assembly of the Protected Head Model. .............................................. 23!

Figure 3.7: The Model is Meshed Using Hexahedral Elements. ..................................... 26!

Figure 4.1: Normal Stress Distribution on the Helmet in the End of the Simulation ...... 35!

Figure 4.2: Visual Illustration of Table 4.1 ...................................................................... 37!

Figure 4.3: Normal Stress Distributions on Skull When the Head is Protected at T=0.1

Sec in The End of Simulation. ................................................................................. 37!


Figure 4.5: Normal Stress Distribution on Skull When Head is Unprotected at T=0.1

Seconds in the End of the Simulation. ..................................................................... 39!


Figure 4.7: Normal Stress Distributions on Brain When the Head is Protected at T=0.1

Seconds. ................................................................................................................... 41!


Figure 4.9: Normal Stress Distributions on Brain When the Head is Unprotected at

T=0.1 Sec. ................................................................................................................ 43!

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Figure 4.10: Visual Illustration of Table 4.5 .................................................................... 45!

Figure 4.11: Visual Illustrations of Von Mises Stresses on The Helmet Model. ............ 45!

Figure 4.12: Visual Illustrations of Von Mises Stresses on Protected Skull. .................. 46!

Figure 4.13: Visual Illustration of Von Mises Stresses on Unprotected Skull. ............... 47!

Figure 4.14: Visual Illustration of Von Mises Stresses on Protected Brain. ................... 47!

Figure 4.15: Visual Illustration of Von Mises Stresses on Brain When Head is

Unprotected. ............................................................................................................. 48!

Figure 4.16: Change in Velocity of Center of Mass of the Head and the Accelerations

Caused by That. ....................................................................................................... 49!

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LIST OF TABLES

Table 3.1: Summary of Material Properties of The Brain Model Used In The FE

Analysis. ................................................................................................................... 29!

Table 3.2: Summary of Material Properties of Helmet Used In The Analysis ................ 31!

Table 3.3: Summary of Physical Properties of All The Models. ..................................... 32!

Table 4.1: Maximum and Minimum Normal Stresses on Helmet. .................................. 36!

Table 4.2: Maximum and Minimum Normal Stresses on Protected Skull. ..................... 38!

Table 4.3: Maximum and Minimum Normal Stresses on Unprotected Skull. ................. 40!

Table 4.4: Maximum and Minimum Normal Stresses on Protected Brain. ..................... 42!

Table 4.5: Maximum and Minimum Normal Stresses on Unprotected Brain. ................ 44!

Table 5.1: Comparison of Maximum Normal Stresses in Two Simulations. .................. 53!

Table 5.2: Comparison of Maximum Von Mises Stresses in Two Simulations. ............. 53!

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LIST OF ABBREVIATIONS

3D Three Dimensional

CSF Cerebrospinal Fluid

COR Coefficient of Restitution

CPU Central processing Unit

CT Computer Tomography

DAI Diffuse Axonal Injury

EDH Epidural Hematoma

EMC Expectation Maximization Classification

FE Finite Element

FEA Finite Element Analysis

FEM Finite Element Method

MRI Magnetic Resonance Imaging

Pa Pascal, a pressure unit

TBI Traumatic Brain Injury

1

Chapter 1

INTRODUCTION

1.1 Need for Head and Brain Modeling in Traffic Accident Analysis

Since the invention of car, car accidents have become a danger to human lives. Every

day men, women and children from all over the world become victims of head injuries,

which can range in intensity from mild concussions to coma and death. Nowadays,

deaths over car accidents have become one of the leading causes of death worldwide.

The estimated brain injury mortality rate is 22-25 per 100,000 people annually (Kleiven,

2002).

Introduction of the new imaging technologies such as magnetic resonance imaging

(MRI) has improved our knowledge of the human head and as a result, medical

treatments of head injuries have become more efficient. On the other hand, with the

increasing rate of brain trauma related fatalities, prevention of such accidents has

become more significant.

There are many theories on how the trauma happens to the brain but almost all the

theories agree that the injury is related to sudden acceleration or deceleration of the

head, even if the impact is not applied directly to the head. The brain is loosely situated

inside the skull so it can be smashed against the interior walls of the skull upon impact.

2

Many studies have been conducted to simulate brain trauma that can occur from head

impact to find the reasons why the trauma happens. Some experimental studies have

been performed by using animals, cadavers and crash dummies to calculate the forces,

accelerations and displacements (Nahum et al. 1977). Using cadavers have many

advantages because they have the same geometry and mass distribution as an alive

person, although experiments on cadavers related to the mechanisms of damage are not

accurate. This is due to some differences in texture between a cadaver and a living body

due to dehydration and decaying of the cadaver. On the other hand, they are not widely

available to everybody due to their costs and ethical reasons.

Development of the Finite Element Method (FEM) brought many advantages to

biomechanical researches. FEM is a numerical technique for finding approximate

solutions to partial differential equations and it can be used to simulate impacts of the

head and to find approximate results for the response of brain to impacts. Introduction of

FEM encouraged researchers to develop more accurate 3D models. The early models

were only simple geometries simulating head and brain, but as the technology improved

more accurate models were created.

There are various ways to create skull and brain models either by using Magnetic

Resonance Imaging (MRI) and Computer Tomography (CT) scan data and extracting the

brain and the skull using the Brain Extraction Tools (BET) available in various software

products (e.g. FSLView manufactured by Oxford University, Mimics and 3D doctor) or

by using 3D printers to scan and model them inside Solidworks and other design

software and even by creating the models from the scratch using Rhino 3D and

3

Solidworks. For the head and brain trauma analysis, there are many software packages

available, including ANSYS, LS-DYNA and ABAQUS.

All the studies previously conducted on impacts to the head suffer from a big problem

and that is the insufficient knowledge we have on human body. As a result researchers

included different material properties for the body parts, which would affect the

performed analysis.

On the other hand, due to lack of technologies and availability of powerful processors,

researchers had to simplify models, and impact conditions and use lower quality meshes

to reduce the processing time. So it is clear that there is still a need to model the human

head tissue more accurately in order to be able to understand the conditions under which

brain trauma happens under lateral impacts and hopefully try to find better ways of

preventing these traumas.

1.2 Anatomy of the Human Head

Human head consists of many parts and each one of them has specific parameters. The

size and shape of these parts vary between individuals. Before going into more details

about the human head modeling, it is adequate to explain the main parts of the human

head as follows:

1.2.1 Scalp

Scalp has 5 different layers which overall are between 5 to 7 mm thick. These layers

are skin, connective tissue, epicranial aponeurosis, loose areolar connective tissue and

4

pericranium. Although the scalp is a very thin part, due to its high elasticity, it helps to

absorb impacts and thus reducing the impact forces that reach the brain.

1.2.2 Skull

Skull is the boney structure of the head, and its thickness varies between 4 to 7 mm. It

has 8 cranial bones and 14 facial bones. All the bones of the skull are joined together so

they have little movement, except the mandible, which can freely move and rotate. Skull

protects the brain from impacts and shapes the head. The brain tissue is so soft and very

vulnerable to impacts and can be easily damaged in impacts and the skull’s role is to

protect it from damaging.

1.2.3 Membrane

Membrane envelops the brain and its job is to protect the brain. It has 3 different layers.

The outermost layer is dura mater. Dura mater is impermeable to fluid, which helps to

enclose CSF. The second layer is called pia mater, which is the innermost layer of the

membranes. The third part is arachnoid mater, which is placed between the other two

layers.

1.2.4 Cerebrospinal Fluid (CSF)

CSF is a colorless and clear fluid that contains proteins and is produced in the

choroid plexus of the brain. CSF protects the brain from bruising into the skull when the

head is hit although it cannot protect the brain in severe hits.

1.2.5 Brain

Brain is the center of the nervous system and it consists of cerebellum, cerebrum and

brain stem. The largest part of the brain is cerebrum and it has two hemispheres and each

5

of them is divided into four lobes the outmost layer of the cerebrum is grey matter and

underneath it is the white matter. Cerebellum is underneath the cerebral hemispheres at

the bottom of the brain. Brain stem is the part where the brain and the spinal cord meet.

1.2.6 Cerebral Vasculature

Cerebral vasculature is the movement of blood through a network of blood vessels.

Major veins in the cranial part are inferior sagittal sinus, the superior sagittal sinus,

cavernous sinuses, straight sinus, transverse sinuses and sigmoid sinuses.

1.3 Head and Brain Trauma in Car/Motorcycle Accidents

The most common cause of death in traffic accidents is head injuries and that happens

when the head is subjected to loads higher than the loading capacities of the head. Head

injuries can be divided into two categories, primary and secondary injuries. If the

consequences of the physical loading of the head happen at the time of injury, they are

called primary injuries, which may result in post-traumatic oedema, hypoxia, increased

intracranial pressures and more. Secondary injuries are the result of the severity of

primary injuries, which may appear minutes from injury up to days after injury and

quick medical treatment can prevent them from happening.

The following are the different variations of primary head injuries that can happen in

traffic accidents:

1.3.1 Skull Fracture

The skull is made of eight bones that form the cranial portion of it. If a blunt force

trauma occurs that breaks one of these bones a skull fracture happens. If the surrounding

6

skin ruptures the skull fracture is considered as an open skull fracture. The skull fracture

can absorb the excess energy applied to the head and transfer less energy to the brain;

however, a significant blow can damage the brain and cause other trauma.

1.3.2 Epidural Hematoma

In traumatic brain injuries (TBI) the damage may cause bone fractures, which can

rupture blood vessels and make the internal parts bleed. If the blood builds up between

the skull and dura mater the condition is called epidural hematoma (EDH). Since the

blood built up between the skull and dura mater can increase the intracranial pressures,

cause brain shift or compress brain tissue, EDH is potentially fatal. According to the

literature about 15 to 20% of EDH are fatal. EDH happens in about 1 to 3% of head

injuries.

1.3.3 Subdural Hematoma

Subdural hematoma or subdural hemorrhage is a type of hematoma happening inside the

skull and is usually the cause of traumatic brain injury. If the blood accumulates between

dura mater and arachnoid mater the condition is called subdural hematoma. There are

three kinds of subdural hematoma according to the speed of their onset: acute, sub-acute

and chronic. The most fatal of all head injuries are the acute subdural hematomas and if

not quickly treated with surgical decompression they have very high mortality rates.

1.3.4 Cerebral Contusion

Contusion is a type of hematoma and if happens to the brain’s tissue, it is called cerebral

contusion and happens to 20 to 30% of severe brain injuries, which often involves

hemorrhage, edema and infraction in the site of the impact.

7

1.3.5 Diffuse Axonal Injury

Diffuse Axonal Injury (DAI) happens when the brain inside the skull is accelerated

specially in cases where the rotation of the head happens. And the cause of DAI is the

traumatic shearing forces that happen when the head is under rapid accelerations.

Almost all the patients with severe DAI go to comma and 90% of them will never gain

consciousness again and between those who wake up, often will get disabled.

1.4 Preventing Head and Brain Trauma by using helmet

Helmet is a protecting device that shields human head from injuries and has many

different applications such as military, vehicular, sports, constructions and etc. What

helmet basically does is to absorb the mechanical energy that is applied to the outer layer

as the energy is transferred towards the head and it is clear that the protective capacity of

the helmet is directly related to the application of the helmet and the impact energy it has

to support.

A motorcycle helmet is a full-face helmet worn by motorcyclist to protect the head from

head injuries. A review in 2008 has shown that motorcycle helmets reduce head injuries

by 69% and death by 42%. There are many types of motorcycle helmets available in the

market. A full-face helmet covers the head entirely and is the safest type of helmet for

motorcyclists. Although avoided by some people because of the excess heat created

inside the helmet and the problems with ventilations. Half-helmets only cover the upper

parts of the skull and face and the chin is still at risk of injury, although there are some

variations that cover the eyes as well.

8

1.4.1 Helmet shell and liner

Helmets are usually consisted of two major parts the outer shell and the inner protective

layer called the liner. The most important part of the helmet in absorbing the impact

energy is the liner and its thickness is usually 3 to 4 cm, and is made up of a kind of

foam.

Hopes and Chinn (1989) investigated the protection power of the helmet with regards to

the stiffness of the liner and the shell. Their result showed that the stiffer the shell and

liners were suitable for higher impacts and accelerations but in lower impacts they could

not handle the impacts efficiently and even at some point they had negative effects on

reducing the head injuries where the degree of the severity of the impacts were low.

Although their study showed that the shell and the liner must be designed stiffly so that

they can protect the head in situations where the head injury is unavoidable.

1.4.2 Fiberglass versus plastic shells

Vallée’s study in 1984 showed that the risk of injury will dramatically rise of the outer

shell fractures and that fiber-based materials where more durable against fracture.

However, Noel in 1979 showed that on the factor of aging, plastic shells were more

resistant against fiber-based materials. But from a safety point of view, fiberglass shells

are preferred to plastic shells.

Organization of the Thesis

This thesis has 6 chapters, the appendix and references. Chapter 1 consists of the

introduction of brain traumas, the biomechanical conditions under which brain trauma

9

happens and how they can be prevented. In Chapter 2 literature survey is conducted and

the development of human head models to simulate car crashes is presented. Moreover,

the creation and development of devices to prevent brain traumas are presented as well.

In this thesis only the skull and the brain will be included in the FE analysis.

In Chapter 3, a FE model of the human head will be developed and later on that chapter

a FE model of the helmet will be developed as well. The results are illustrated in Chapter

4 and discussed in Chapter 5. Finally, in chapter 6 conclusion and suggestion for future

work are presented.

10

Chapter 2

LITERATURE REVIEW

2.1 Head Injury Prediction

Many experimental tests have been performed since early 90s to predict the damages to

the head in different circumstances like car crash, falling and etc.

One of the most commonly used methods is Head Injury Criterion (HIC), an improved

method of Wayne State Tolerance Curve (WSTC). WSTC was first introduced by

Lissner in 1960, which described the link between the impact of the head caused by

acceleration, period of the impact and the risk of head injury. Gadd (1966) introduced a

new method of estimating injury hazard. He developed a Severity Index (SI) number to

improve the impact tests by defining how severe is a head trauma.

The National Highway Traffic Safety Administration (NHTSA) used Gadd’s work to

introduce the Head Injury Criterion (HIC) in 1972, which is still being used in head

protection systems using head forms. There are some limitations for the HIC according

to Newman (1986), the direction of the impact is not specified in HIC and angular

accelerations are not considered as well. So the Generalized Acceleration Model for

Brain Injury Threshold (GAMBIT) was introduced by Newman, and in the late 1990s

Head Impact Power (HIP) was introduced by him as well.

Although rotational accelerations have a massive effect on brain injuries, HIC only

11

considers linear accelerations and that’s why Willinger (2008) developed an improved

head injury criteria based on his previously created FE model.

2.2 Development of Head, Neck and Torso Models for Simulating

Car/Motorcycle Crashes

The development of the FE method brought many advantages to biomechanics

researchers. The FE method is a numerical technique for finding approximate solutions

to partial differential equations. In the past two decades various researchers have

introduced many FE models of the head. The first geometrically similar to the head FE

model, was introduced by Hardy and Marcal (1971) and it was used for static

simulations. Nickell & Marcerl (1974) developed a model containing only the skull.

In 1977, Shugar presented a new model that was used for head impact simulations.

Until now, many different FE models have been introduced which differ in complexity

and accuracy. Mendis, (1992) considered rotational acceleration of the head and

developed a FE model to analyze stress and strain of such accelerations.

Nahum et al., (1977) conducted an experimental test using human cadavers for frontal

blow to the head. The blow to the head was done using a cylindrical impactor with

padding. The final results showed the epidural pressures in different locations of the

head.

Ruan et al., (1993) presented a model that was geometrically based on Shugar’s model

12

consisting of the scalp, skull, brain, dura matter and the falx cerebri. The simulation

setup of the analysis was based on cadaver tests of Nahum. He considered the brain as a

viscoelastic material in his study. Ruan also did a similar study but with different

material properties of skull, brain and cerebrospinal fluid (CSF) which affected the

intracranial pressure levels and stress and strain levels of the skull.

Kumaresan et al., (1995) used the geometry of a dummy head (Hubbard, 1974) and

created a FE head model containing the skull, CSF and brain and he approximated the

mesh with his own software. The final model had 293 shell elements for the skull and

555 brick elements for the CSF and brain.

During the same year, Krabbel and Müller introduced a new way of creating FE models

of head using visible human data extracted from CT and MRI scans. The inner and outer

layers of the skull were created from CT scan data and brain model was extracted using

MRI data. They scaled down the pixel sizes from 0.9 mm to about 0.5 mm to be able to

use fewer elements for the model to make the analysis easier. The final model had 9,732

brick elements for the brain and the skull. The study resulted in creating FE models in a

much easier and more accurate way.

In 1995 Zhou et al. created a new more detailed FE model, which was based on the

model developed by Ruan but in addition it contained grey and white matter, the

ventricles and the bridging veins and also finer meshing was used for the brain. All the

parts were considered to be linear elastic. The impact in the simulation was the same as

Nahum’s experimental impact but the model was subjected to angular acceleration pulse

13

as well.

Ruan et al., (1997) conducted a new study that again Nahum’s impact experiment was

used in the simulation and the comparison of the results with HIC values verified the

accuracy of the results.

In the same year, Eindhoven University of Technology created a CT and MRI scan

based model consisting of the brain, skull and facial bone. (Claessens et al., 1997). They

used the projection method to generate the mesh for the brain and by expanding the

meshes of the brain they meshed the outer layer of the skull as well. Later, the CSF, falx,

dura mater and tentorium were added to the model (Brands et al., 2002) and the model

was validated against Nahum’s experiments. (Nahum et al., 1977)

Wayne State University researchers developed a model of the head in 1993 called the

Wayne State University Brain Injury Model (WSUBIS). Later, they upgraded the model

and release several versions of the model. The first version of the model (1993-1997)

includes the essential parts of the human head. The total mass of the model is 4.3 kg.

The model can predict diffuse axonal injury (DAI) in porcine brain because the grey and

the white matter have different material properties. The second version of the model

(1998-1999) was an improved version of the first model.

In the new model the brain can slide inside the skull, which improves the accuracy of

brain trauma results in direct, and indirect head trauma caused by accelerations and

deceleration of the head.

14

In 1997, Kang developed a new FE model of the human head, which is known as

Université Louis Pasteur (ULP) model. In 2004, Deck et al. developed a new modified

version of the ULP model including a very detailed geometry of the skull, considering

different thickness throughout the skull. And for the first time the model included the

reinforced beams in the skull, which had a significant role in modeling the skull fracture.

McGuffie et al (1998), studied golf related head trauma on children. In their results they

showed the effects of the velocity of the ball on the trauma caused to the head.

In 2002, Kleiven and Hardy developed a new head model consisting of the scalp, skull,

brain, meninges, cerebrospinal fluid and eleven pairs of parasagittal bridging veins based

of the visible human data. And a simplified neck model was introduced as well with the

spinal cord, dura matter, spine, muscle and skin. The model has 18400 elements.

In 2003, Horgan and Gilchrist created a model (The University Collage Dublin Brain

Trauma Model (UCDBTM)) based on CT scan data, which was used to model the

pedestrian accidents. Using interpolations and threshold schemes smoothed the skull to

create a FE model. To mesh the CSF and brain, the inner surface of the skull was used.

An improved version of the model was introduced a year after consisting of the grey and

white matter in addition to other parts. The model then validated against localized brain

motion experiments (Hardy et al., 2001 and cadaver impact tests (Horgan and Gilchrist,

2004).

Using willinger’s model (1999), Zhang et al (2004) presented the injury threshold for

mild brain injuries. Zong, et al (2004), analyzed impact power flow on a 3D FE head

15

model. Gong, et al (2004), modeled a solid impact to human head model and analyzed

the response of the head to the impact.

In 2006, Kimpara et al. developed a 3D FE model with a highly detailed brain and spinal

cord geometry. The model was used to simulate the entire central nervous system’s

mechanical behavior in impacts to the head simultaneously

In the same year, Paul and Corey (2006) used CT scans of a healthy female head and

segmented the skull, brain and CSF. Each axial plane had a high resolution but the

resolution between the planes was low. The details of the head model was not mentioned

nor validated against other studies. In their study they developed their own version of the

head model to use on their future studies.

2.3 Modeling and Analysis of Head Trauma During Traffic Accidents

Vallée et al (1984) showed in their study that the risk of injury increases if the helmet

breaks and in helmeted head impacts fiber materials were more durable than plastic

material against fractures. However in Noél’s study (1979) which was about aging of

materials that showed plastic materials were more durable than fiber materials so

Vallée’s results were true only in safety point of view.

Köstner (1987) and Van Schalwijk (1993) described the early helmet models but none of

them were validated. Helmet models are usually modeled as linearly elastic although in

some researches the helmet was considered as rigid.

16

Hopes and Chinn (1989) used different stiffness for the helmet shell and the liner in their

study and the results showed that stiffer shell or liner was better for high accelerations.

Yettram (1994) developed a simplified FE model of the helmet. The helmet had only

one layer and a wooden spherical head was used inside the helmet. The result showed

that the density of the liner and the stiffness of the helmet had an extensive effect on the

performance of the helmet and the lower the density, the higher the performance of the

helmet.

Brands (1996) created a 3D FE model and used a dummy for the head and used elastic

material for the shell and an elasto-plastic material for the liner, which was glued to the

helmet. This model could not handle lateral impacts and it was used for frontal impacts

only.

Liu (1998) simulated the head trauma using a FE model of helmet and again the model

had only one layer. For the simulation process a biomechanical human head (the FE

model of the Hybrid III dummy) including the neck was used. The results of that

research were acceptable and were validated against other works.

Chang (1999) used a more detailed model for the helmet and included a rigid head

model. The result was that higher stiffness of the helmet and a more dense liner was

better for high-speed impacts and vice versa.

Chang (2000) conducted another simulation using chin bar and an energy absorbing

17

liner which improved the safety of the helmet in facial impacts and showed that a less

stiff helmet with the combined chin bar and energy absorbing liner could be much safer

in crashes.

Willinger (2000) calculated the response of the head under normative impacts. The von

mises stress and the pressure of the brain under the impacts was calculated and was

validated against Nahum’s cadaver head impacts.

Kotsiopoulos (2002) developed their helmet model using composite materials, which

showed that composite shell helmets were absorbing more energy during impacts and

were more reliable during crashes.

Aare and Halldin (2003) modeled a plastic shell helmet with elastic material and the

liner was energy absorbing expanded polystyrene and was validated against oblique and

radial impacts.

Pinnoji et al. (2007) combined Remy’s head and brain model with a FE model of the

helmet and calculated the force applied to the head during an impact with and without

the helmet to determine the effects of the helmet in absorbing the force transferred to the

head during frontal impact. The FE model used in the study had two layers and they

monitored the results when the thickness and density of the liner were changed to find

how it affects the impact forces and pressures absorbed by the helmet and transferred to

the head.

18

Hamouda et al (2007) modeled the helmet and for the liner’s material used expanded

polypropylene and showed that using expanded polypropylene would increase the

energy absorption of the liner.

Callahan (2011) analyzed a composite FE head model and for the head a Roma

Plastilina #1 clay-filled rigid and stationary headform was used. The study showed the

number of the layers of the helmet and the crated affected the results and predicted that

the crater size in the clay have an important role in the head injury caused by the trauma

to the helmet.

Through the literature survey, it is clear that there is still a need to work further on

lateral impacts to the head and there is no need to mention that most of the previous

projects were emphasizing the head only and the effects of the helmet were not

analyzed. Very few FE helmet models are reported in the literature. The previous

projects that were focused on helmet basically considered a rigid head form inside the

helmet and only the effects of the helmet on reduction of the impact forces were

analyzed, in other cases however, the helmet was simplified to a rigid body or a one part

body.

19

Chapter 3

MATERIALS AND METHODS

3.1 Construction of 3D Models

In this thesis, Solidworks is used to create 3D models of brain, skull and helmet. All the

parts are designed in Solidworks and the measurements are taken from available data.

The methods, by which these parts are created, are explained as follows.

3.1.1 3D Model of the Helmet

To create the helmet, in a 2D sketch, a half circle with the diameter of 10 cm is created

and with the Revolved Extrude feature a half sphere is created shown in Fig. 3.1.

Figure 3.1: A Half Sphere Created as a Part of Helmet Model

20

In the next step, from the center of the sphere, a Boss Extrude with the same diameter of

the half sphere is created at 15 degrees angle with the length of 5.99 cm (see Figure 3.2).

Figure 3.2: A boss extrude with 15 degrees angle is created from the center of the sphere

Once the second extrusion is completed, a fillet with a 4.5-cm diameter between the two

extrusions is created and by using the Shell feature, the model is transformed from a

solid body to a shell with the diameter of 2.2 cm, which is the thickness of the helmet

according to the literature.

21

Figure 3.3: The helmet model is converted into a shell In the final step, a surface cut is made in the center of the model to create the visor of the

helmet. And in the edges of the visor a 1.27-cm radius fillets are created as well (see Fig.

3.4).

Figure 3.4: Final Geometry of the Helmet Model After a Surface Cut is Performed.

3.1.2 Brain and Head

For the scope of this thesis, accurate models of brain and head are not needed. So for the

brain and the helmet, simple spheroids are used instead. Although for the skull,

22

according to the literature thickness of 7 mm, which is the mean thickness of the skull, is

used. Finally both brain and skull are assembled in Solidworks using the Mate feature.

(See Fig. 3.5)

Figure 3.5: Assembly of the Skull and Brain Models

3.1.3 Final Assembly

For the assembly, all the parts are either scaled down or up in order to fit in the

assembly. Figure 3.6 shows the final assembly created for the analysis. The impact

object is situated very close to the helmet in order to reduce duration of the FE analysis.

23

Figure 3.6: Final Assembly of the Protected Head Model.

3.2 Developing a Finite Element Model of Head and Brain

3.2.1 Meshing

Meshing the model correctly is one of the most important parts in FE analysis. A bad

mesh can affect the final results dramatically and on the other hand, it can increase the

processing time, CPU usage and memory usage. Reducing the number of the elements in

the mesh is a very good way to reduce the processing time of the analysis and there are

many ways to do that, either by increasing the element size or by fixing the 3D CAD

model, such as omitting the fillets or refining the sharp edges. In the meshing, there are

two important factors that if chosen correctly can make the analysis much easier:

24

3.2.1.1 Element types

Choosing a correct element type is crucial to the accuracy of the final results in FE

analysis. According to a couple of studies that were focused on how choosing element

type woud affect the final results, usually use of the linear tetrahedral elements is

avoided in biomechanical analysis because they can affect the results for nearly

incompressible materials and the reason to that is that it causes volumetric locking and a

stiffed bending responses. (Bonet et al., 2000)

Some studies have shown that linear hexahedral elements are much more accurate and

have a much better performance in CPU time and accuracy than even quadratic

tetrahedral elements. (Benzley et al. 1995)

Brain includes high amounts of water in itself and because of that is considered to be

nearly incompressible (Poisson’s ratio, υ ≈ 0.5) so using hexahedral elements for the

brain is much more efficient in the analysis of brain injuries.

For the skull, using shell elements has better accuracy and lowers CPU time and studies

have shown that using tetrahedral shell elements for the skull is more efficient in impact

analysis.

Triangle meshes are avoided wherever possible and almost all the parts have quad type

meshes.

25

3.2.1.2 Element quality

Errors in FE simulations are directly related to element quality. Large deformations in

element will reduce its quality. The accuracy of the FE problems will reduce if the

meshing of the part has large amounts of deformed elements.

Evaluating the quality of the element from its shape and dimension is possible using a

variety of methods.

Jacobian value is a method to calculate the difference between an ideal element and the

deviated element. The range of the Jacobian value is between 0 and 1 and usually values

above 0.5 are considered as an acceptable deviation for the element. Another method for

measuring the element’s quality is measuring its aspect ratio. The ratio of the longest

side to the shortest side of the elements must be less than 4:1 and in areas with quick

changes in stress the ratio should be much smaller. Measuring the angle between the

lines that join opposite sides is another method of determining the quality of the element.

The aforementioned angle is called skew angle.

In the current thesis the meshes satisfy all the above methods. Figure 3.7 shows the

meshed model in ANSYS. The meshes for the skull, brain and helmet are converted to

hexahedrons by using the Hex Dominant Method.

26

Figure 3.7: Hexahedral Elements for Meshed Model.

The model is meshed in ANSYS, and brain, skull and helmet have 493, 1367 and 2502

nodes and 2008, 4085 and 10404 elements respectively.

3.2.3 Material properties

Biological tissues are considered to be viscoelastic, nonlinear, anisotropic and

inhomogeneous, but generally for analysis they are assumed to be linearly elastic. In this

thesis, Material properties assigned to the different parts are all isotropic, homogenous

and elastic.

3.2.3.1 Brain

With very large amount of water in the brain, it is assumed to be nearly incompressible

with a poisson’s ratio very close to 0.5. According to the literature the density of the

27

brain is about 1.04 kg/dm3. The bulk modulus of the brain is close to water’s bulk

modulus and about 2.1 GPa (McElhaney et al., 1976; Stalnaker, 1969). Although in the

brain, grey matter and white matter have different material properties but in the scope of

this thesis they are neglected.

Although older studies considered the brain as a viscoelastic material in newer studies

Mooney-Rivlin hyper elastic rubber like material model is one of the models that are

used to describe brain’s behavior (Hallquist, 2006). Marvern, 1969 has shown that the

stored strain energy for a hyper elastic material and in our case the brain is a function of

the principal invariants (I1, I2, I3) of the right Cauchy-Green deformation tensor. That is

W=W(I1, I2, I3). Although this formula is implemented in ANSYS and only the constants

in Table 3.1 are needed in order to use this formula.

! ! !!, !!, ! = !!" !! − 3 + !!" !! − 3 + 12!(! − 1)!!!!! (3.1)!

! !! = !!!!!! !! (3.2)!

! !! = !!!!! ! ! (3.3)!

I1, I2, I3 are the strain invariants of the right Cauchy-Green deformation tensor.

J is the Relative volume.

C10, C01 are the Moony-Rivlin constants.

K is the Bulk modulus

W is strain energy density function and by inputting the constants to ANSYS equation

3.1 will be used to analyze the brain.

28

Since we cannot directly input the Poisson’s ratio into ANSYS, the following formula is

used to derive the incompressibility parameter from Poisson’s ratio:

! = !3(1− 2!)

(3.4)

! = 2/! (3.5)

So we have:

! = 6(1− 2!)!

(3.6)

Where:

K is the Bulk modulus

E is the Young’s modulus

ν is the Poisson’s ratio

In the analysis, the bulk modulus of the brain is considered to be 2.19 GPa. Poisson’s

ration of the brain is assumed to be 0.4996, which resembles a material nearly

incompressible. Using the equation (3.4), the young’s modulus of the brain is calculated

5.26 MPa and by putting the result in equation (3.6), we will find the incompressibility

factor to be 9.125E-10.

Table 3.1 summarizes inputs that are used in ANSYS

29

Table 3.1: Summary of material properties of the brain model used in the FE analysis.

Tissue Material properties Density Kg/m3 Poisson’s ratio

Brain

Mooney-Rivlin rubber

C10=124

C01=138

d=9.125E-10

1040

0.4996

3.2.3.2 Skull

Generally, skull is assumed to be linearly elastic. Density of the skull is about 1.3

Kg/dm3 and the Young modulus of the skull is about 15 GPa. Poisson’s ratio of the skull

is about 0.22 (McElhaney et al., 1970). Skull has different thicknesses in different parts

and to make the analysis easier the mean thickness of the skull is assumed to be 7 mm.

For the analysis, bulk modulus of the skull is needed, which is calculated from (3.4)

equation and is 8.93 GPa. The total weight of a human head differs in different ages and

genders and in adults it varies between 4 to 6 kg. The head model, which is used in this

thesis, is weighed 5.4 kg.

3.2.4 Contacts between brain and skull

In the literature review (Sec. 2.2), 3 types of contacts are defined between the skull and

the brain. The first method is to bond the skull and the brain together, so that there is no

relative movement between the two.

30

The other method is to consider frictionless movement between brain and skull and the

third method is to consider a frictional movement between the parts and considering that

the brain is drowned in the CSF inside the skull this method is more realistic than the

others.

In this thesis, the brain has a frictionless movement inside the skull and the reason to

choose this method is that CSF is outside the scope of this thesis.

Aside from contacts, the interactions between all the bodies are considered to be

frictionless as well.

3.3 Finite Element Model of Head and Brain with Helmet

In the second part of the analysis, effects of the helmet on reducing the stresses reached

to the brain during the impact to the head will be analyzed. In this thesis, the helmet

consists of only one part, the inner liner. According to the literature the most important

part of the helmet in absorbing the impact forces is the liner. The helmet used in this

study is a full-face helmet, covering all the parts of the head. The liner is made of

expanded polystyrene foam. The thickness of the helmet is 40 mm.

The helmet model is meshed with quadrilateral elements, which brings acceptable

results and reduces the processing time needed for the analysis. The total mass of the

helmet is 0.3 Kg and its low weight is because the outer shell is not considered in this

thesis. The head model used for this part of the thesis is the model introduced in section

3.1. Material properties of the helmet are shown in Table 3.2 and physical properties of

the FE model are given in Table 3.3.

31

Table 3.2: Summary of material properties of the helmet used in the analysis

Material Model Young modulus

[Gpa]

Poisson’s

ratio

Density

[Kg/m3] Comment

Expanded

polystyrene

Elasto-

plastic

1.5e-3

0.05

0.25

Thickness 40

mm

Yield stress =

0.35 MPa

32

Table 3.3: Summary of physical properties of all the models.

Object Name Impact Object Brain Skull Helmet

State Meshed

Definition

Stiffness Behavior Rigid Flexible

Properties

Volume 1.2233e-003 m³ 2.4658e-003 m³ 1.2683e-003 m³ 1.1785e-

002 m³

Mass 3.0582e-002 kg 3.5902 kg 1.8467 kg 0.29462

kg

Centroid X 0.17963 m 5.4676e-003 m 5.9034e-003 m 5.0068e-

003 m

Centroid Y 4.493e-002 m 6.789e-002 m 1.9108e-002 m 5.3139e-

002 m

Centroid Z 1.495e-002 m -1.6557e-002 m -1.6297e-002 m -3.1897e-

002 m

Statistics

Nodes 1160 493 1367 2502

Elements 532 2008 4085 10404

3.4 Impact conditions and Finite Element analysis of the model

A very important factor affecting the intensity of injuries during the impact is weather

the head is able to move freely after it is hit or not. If the head cannot move freely after

the impact the injuries are directly related to the location where the head hits an object

33

and how the skull deforms. However in most cases the head impacts a stationary object

and because of that the injuries are mostly due to the acceleration of the head. To

simulate the impact of the head, a rigid object has been used to hit the head laterally. The

velocity of the impact is assumed to be 4 m/s as specified by the standard. The duration

of the analysis is 0.1 sec.

Coefficient of Restitution (COR) for skull and brain are considered to be 1. The reason

behind that is because in this thesis skull and brain are in the elastic zone so the collision

of the objects is considered to be elastically. However, for the helmet, COR is

considered to be 0.45.

The impact analysis of the head is performed twice. In the first simulation the head is not

protected and it laterally hits the impact object. However, in the second simulation the

head is protected with the helmet model and the same impact conditions are applied to

the model. Normal and Von Mises stresses for both simulations are calculated and in the

end compared with each other in order to find out about the effects of the helmet in

reducing such stresses.

34

Chapter 4

RESULTS

The most common injury in car and motorcycle accidents is head injury (Otte et. al.,

1999). Although in many countries using helmets on motorcycles are obligatory there is

still a need to improve the helmet design to prevent head injury. In this thesis a FE

analysis is conducted to simulate the lateral impact to head that may occur in car

accidents then normal and equivalent stresses that are applied to the brain during such

accidents have been analyzed. In this analysis the head is subjected to the impact twice.

In the first analysis the head is not protected and the impact directly hits the skull at the

speed of 4.5 m/s.

In the second analysis the head is protected with a helmet and again it is subjected to the

same impact. These two analyses are performed separately and in Chapter 5 the results

are compared to find out the effect of the helmet in absorbing external impacts to the

head.

The results in the present chapter represent the stresses that are created during the impact

of the head and the helmet to the impact object. Normal Stresses are shown in Section

4.1, which is divided into 3 subsections: Normal stresses on Helmet, skull (Protected

with helmet and unprotected) and Brain (protected with helmet and unprotected) are

35

given in Subsections 4.1.1, 4.1.2 and 4.1.3 respectively. Afterwards in Section 4.2 Von

Mises stresses are calculated and given which is again divided into 3 Subsections as in

4.1.

4.1 Normal Stress Analysis

Normal stresses occur as a result of vertical internal and external forces. In this section

normal stress distributions on different parts of the model under the lateral impact are

illustrated. In all the figures green lines are Maximum stresses and red lines are

minimum stresses. Minimum stresses happen due to internal forces rather than

maximum stresses, which are the result of external forces. In the case of skull minimum

stresses and in the case of brain maximum and minimum stresses are due to intracranial

forces.

4.1.1 Helmet

Stresses on helmet during the impact of the impact object to the head at t=0.1 which is

the end of the analysis are shown in Figure 4.1.

Figure 4.1: Normal Stress Distribution on

the Helmet in the End of Simulation

36

Maximum and minimum stresses for the duration of the impact are shown in Table 4.1

and are illustrated in Figure 4.2.

Table 4.1: Maximum and Minimum normal stresses on helmet due to impact.

Time (s) Minimum (Pa) Maximum (Pa)

1.1755e-038 0. 0.

5.0009e-003 -29134 1620.2

1.e-002 -23835 2696.3

1.5e-002 -17385 8589.6

2.5e-002 -60138 4491.1

3.e-002 -60490 14693

3.5001e-002 -64673 9861.8

4.5e-002 -31637 4907.

5.0001e-002 -10348 4658.5

5.5e-002 -34763 10859

6.e-002 -66679 20529

6.5001e-002 -41939 18118

7.e-002 -42148 14733

7.5e-002 -36995 14017

8.0001e-002 -10853 12066

8.5001e-002 -10941 9911.6

9.e-002 -21676 11435

9.5001e-002 -10805 10786

0.1 -11884 11064

37

Figure 4.2: Visual Illustration of Table 4.1

4.1.2 Skull

4.1.2.1 Head protected with helmet

Normal stresses on skull during the impact when the head is protected with the helmet in

the end of simulation (t=0.1) are shown in Figure 4.3.

Figure 4.3: Normal Stress Distributions on Skull When Head is Protected at t=0.1 Sec in

the End of Simulation.

38

Maximum and minimum normal stresses on the skull are given in Table 4.2 and are

illustrated in Fig. 4.4.

Table 4.2: Maximum and Minimum normal stresses on protected skull.

Time [s] Minimum [Pa] Maximum [Pa]

1.1755e-038

0. 0. 5.0009e-003

1.e-002

1.5e-002 -1.1539e+005 1.7681e+005

2.0001e-002 -51624 33854

2.5e-002 -78161 1.5325e+005

3.e-002 -1.3959e+005 1.1142e+005

4.e-002 -1.0221e+005 47430

4.5e-002 -1.2137e+005 80595

5.0001e-002 -49656 52805

5.5e-002 -67289 81243

6.e-002 -84051 89508

6.5001e-002 -1.344e+005 1.0954e+005

7.e-002 -1.1095e+005 1.2741e+005

7.5e-002 -54918 1.0417e+005

8.0001e-002 -1.1229e+005 1.4438e+005

8.5001e-002 -1.6724e+005 1.8522e+005

9.e-002 -1.5342e+005 1.4723e+005

9.5001e-002 -1.2327e+005 1.2068e+005

0.1 -84973 1.9882e+005

39

Figure 4.4: Visual illustration of Table 4.2

4.1.2.2 Unprotected head

Stresses in skull when the head directly hits the impact object without any protections in

the end of simulation (t=0.1) are illustrated in Figure 4.5

Figure 4.5: Normal Stress Distribution on Skull When Head is Unprotected at t=0.1

Seconds in the End of the Simulation.

40

Maximum and minimum stresses on the skull are given in Table 4.3 for the duration of

the impact and are illustrated in Fig. 4.6.

Table 4.3: Maximum and Minimum normal stresses on unprotected skull.


1.1755e-038 0. 0.

5.0031e-004

1.0001e-003 -3.2453e+006 3.6802e+006

1.5003e-003 -7.173e+006 6.7372e+006

2.0001e-003 -2.4408e+006 7.4586e+006

2.5003e-003 -1.9674e+006 8.0699e+006

3.5e-003 -1.8227e+006 6.306e+006

4.0003e-003 -1.5552e+006 5.902e+006

4.5001e-003 -1.0347e+006 3.6825e+006

5.0003e-003 -1.2074e+006 3.5357e+006

5.5002e-003 -8.7079e+005 2.9168e+006

6.e-003 -7.5275e+005 1.8943e+006

6.5003e-003 -1.343e+006 1.5483e+006

7.0001e-003 -9.7209e+005 1.8506e+006

7.5003e-003 -1.6092e+006 3.3211e+006

8.0001e-003 -1.2411e+006 4.0006e+006

8.5003e-003 -1.4162e+006 3.9278e+006

9.0002e-003 -1.038e+006 3.775e+006

9.5e-003 -1.1279e+006 3.105e+006

1.e-002 -7.7709e+005 2.1421e+006

41


4.1.3 Brain

4.1.3.1 Head protected with helmet

Stress distribution on brain in the end of simulation (t=0.1) when head is protected with

a helmet and hits the impact object are shown in Figure 4.7.

Figure 4.7: Normal Stress Distributions on Brain When the Head is Protected at t=0.1

Seconds.

42

Maximum and minimum normal stresses are given in Table 4.4 and are graphed in Fig.

4.8.

Table 4.4: Maximum and Minimum Normal Stresses on Protected Brain.


1.1755e-038

0. 0. 5.0009e-003

1.e-002

1.5e-002 -87815 3276.4

2.0001e-002 -6875.1 3572.2

2.5e-002 -63388 6910.7

3.e-002 -50352 5745.4

3.5001e-002 -22422 14584

4.e-002 -34967 9167.1

4.5e-002 -29911 20096

5.5e-002 -23865 17002

6.e-002 -50338 27652

6.5001e-002 -57520 67737

7.e-002 -41018 17844

7.5e-002 -76205 10132

8.0001e-002 -32773 18551

8.5001e-002 -74761 12230

9.e-002 -93927 25857

9.5001e-002 -46124 19427

0.1 -35424 21991

43



Stress distributions on brain when head is unprotected and it directly hits the impact

object in the end of simulation (t=0.1) are shown in Figure 4.9.

Figure 4.9: Normal Stress Distributions on Brain When Head is Unprotected at T=0.1

Sec.

44

Maximum and minimum stresses are given in Table 4.5 and are graphed in Fig. 4.10.

Table 4.5: Maximum and Minimum Normal Stresses on Unprotected Brain.


1.1755e-038 0. 0.

5.0031e-004

1.0001e-003 -2.8358e+006 21124

1.5003e-003 -4.4991e+006 1.5009e+005

2.0001e-003 -4.6591e+006 1.242e+005

2.5003e-003 -4.9683e+006 1.6106e+005

3.0002e-003 -4.4624e+006 1.3493e+005

4.0003e-003 -3.5792e+006 83043

4.5001e-003 -5.6506e+005 5.0375e+005

5.0003e-003 -5.5702e+005 53281

5.5002e-003 -4.8186e+005 2.4012e+005

6.e-003 -4.2315e+005 1.3993e+005

6.5003e-003 -5.0887e+005 2.4279e+005

7.0001e-003 -4.8586e+005 3.6007e+005

7.5003e-003 -6.533e+005 1.1338e+005

8.0001e-003 -1.0526e+006 2.581e+005

8.5003e-003 -8.1239e+005 1.0483e+005

9.0002e-003 -6.1444e+005 2.36e+005

9.5e-003 -5.1395e+005 54944

1.e-002 -4.1335e+005 1.8665e+005

45


4.2 Von Mises Equivalent Stress

In this section Von Mises equivalent stress distributions, which according to Willinger,

R, 2000, are considered as good indicators of concussion, are illustrated for the duration

of the impact.

4.2.1 Helmet

Von Mises stresses on the helmet during the impact are depicted in Figure 4.11.

Figure 4.11: Visual Illustrations of Von Mises Stresses on the Helmet Model

46

4.2.2 Skull

4.2.2.1 Head is protected with helmet

Von Mises stress distributions on skull during the impact when head is protected with

helmet are illustrated in Figure 4.12.

!Figure 4.12: Visual Illustrations of Von Mises Stresses on Protected Skull


Von Mises stress distributions on skull during the impact when the head is

unprotected and it directly hit the impact object are illustrated in Figure 4.13

47

!Figure 4.13: Visual Illustration of Von Mises Stresses on Unprotected Skull

4.2.3 Brain

4.2.3.1 Head is protected with helmet

Maximum and minimum Von Mises stress distribution on the brain when head is

protected with helmet for the duration of the impact are illustrated in Figure 4.14.

Figure 4.14: Visual Illustration of Von Mises Stresses on Protected Brain.

48


Von Mises stress distribution on brain when the head is unprotected and it directly hits

the impact objects are shown in Figure 4.15.

!Figure 4.15: Visual Illustration of Von Mises Stresses on rain When Head is

Unprotected.

49

4.3 Acceleration on Center of mass of the head

Differences between changes in velocity of mass center of the head during the two impacts are shown in Figure 4.16

!Figure 4.16: Change in Velocity of Center of Mass of the Head and the Accelerations

Caused by That.

These results are compared in Chapter 5 in order to understand how helmet helps to

protect the head from lateral impacts.

50

Chapter 5

DISCUSSION

In this chapter the results that have been obtained from analyses are discussed. The

results of this study imply than helmet is very effective in reducing effects of external

impact forces during lateral head trauma, which have a direct effect on stress

distributions on the head.

In this study several limitations are included in order to reduce the processing time and

to prevent using extra variations. These limitations include:

o The at which the head model hits the impact object was kept low at 4.5 m/s to

eliminate the unwanted oscillations of brain inside the skull and in order to omit

extra factors and achieve more accurate results.

o The models of the skull and brain were simplified to reduce the meshes and

increase the quality of the meshes that were created.

o Helmet straps are neglected although it is very unlikely that straps have

considerable effects on the results of this study.

o The helmet model only consists of the liner as other studies have shown liner is

the most effective part of the helmet during impacts.

51

o The skin was excluded from this study as it has a minimal effect in reducing the

impacts because of its low thickness around the skull.

o The interactions between brain and skull were considered frictionless to reduce

the complexity of the analysis.

5.1 Discussion of the Results

Simply by comparing the results in two cases (protected and unprotected), it is apparent

that helmet will effectively reduce the impact forces, which results in reduction of stress

in the brain (Figures 4.8, 4.10) and the skull (Figures 4.4, 4.6). According to the results,

the maximum normal stress on brain in an unprotected head is about 4.97 MPa which is

as a result of intracranial pressures, comparing to result of the protected case, which is

93.9 KPa. So there was a 98% reduction in maximum stress on the brain when the

helmet was used. Moreover, in the skull maximum stresses on the unprotected case and

protected case were about 8 MPa and 199 KPa respectively, which has reduced

maximum normal stress on the head by 97.5%.

Although the skull was not damaged in neither of the cases but the stresses on the brain

in the unprotected case could be lethal according to the literature where the critical

stresses on the brain were considered as 100 KPa. So the protected head is in the safe

zone and it is highly unlikely that the impact would cause brain trauma.

We can see that minimum stresses, which are caused by internal forces that are created

inside the head have higher absolute magnitudes so in normal stress they are used

instead of maximum stresses which are as a result of external forces.

52

On the other hand, the helmet did not fail during the impact because according to the

literature the failure of the helmet will probably have negative effects in protecting the

head from trauma.

In this study Von Mises stresses were calculated as well. The reason behind that is

according to the definition of Von Mises Criterion although stresses on the X, Y and Z

direction may not cause failure in an object but the combination of these stresses may

cause failure. So Von Mises stresses were also calculated to assure that combination of

the stresses would not cause trauma in the protected head. According to the results

maximum Von Mises stresses on the brain in unprotected and protected cases were 8.45

MPa and 248 KPa respectively which shows a 97% reduction in the maximum Von

Mises stress on the brain. For the skull, the results were 34.6 MPa and 457 KPa

respectively, which reduced the maximum Von Mises stress on the skull by 98%.

It should be mentioned that since absolute values of Minimum Von Mises stresses were

considerably lower, maximum Von Mises stresses were used instead. Table 5.1 and 5.2

summarize these comparisons.

53

Table 5.1: Comparison of Maximum normal stresses in two cases.

Parts of the head Maximum normal

stress on protected

head with helmet

(KPa)

Maximum normal

stress on

unprotected head

(KPa)

Percentage of

stress reduction

(%)

Brain 93.9 4970 98.1

Skull 199 8000 97.5

Table 5.2: Comparison of Maximum Von Mises stresses in two cases.

Parts of the head Von Mises Stress

on protected head

(MPa)

Von Mises Stress

on unprotected

head (MPa)

Percentage of

stress reduction

Brain 0.248 8.45 97

Skull 0.457 34.6 98

By comparing the all the figures related to normal and Von Mises stresses for

unprotected and protected cases, it is clear that in protected cases although the maximum

levels of normal and Von Mises stresses were lower but there were more fluctuations

compared to unprotected cases. The reason behind this is because of the elasticity of the

helmet liner, which has caused these fluctuations, and by considering the outer shell for

the helmet these fluctuations would be either prevented or reduced dramatically.

54

Another important factor in two cases is the change in velocity of the mass center of the

head after the impact (Section 4.3). According to figure 4.16 the total change in velocity

of the protected head is nearly 80% of the unprotected head. Although this change is not

significant but the duration under which this change happens is much higher than that of

the unprotected head. This will result in lower acceleration of mass center of the head.

According to Hutchinson. J., 1998, accelerations higher than 1000 g, increase the risk of

brain trauma. Maximum value of mass center of the head acceleration is 180 g for

protected head and 1064 g for unprotected head. So a considerable amount of the energy

is absorbed in the protected head.

The results in both sections (4.1 and 4.2) were close to the literature (Afshari A. et al.,

2008) with less that 15% difference, which is because of the simplified geometries,

different material properties and different direction of the impact to the head (Lateral

impact instead of frontal impact).

5.2 Key Research Accomplishments

o A comprehensive literature survey on head and helmet impacts

o Application of FEM for analyzing head trauma in two different cases (Head

protected with a helmet and unprotected head).

o Determination of the importance of the helmet in reducing the normal and Von

Mises stresses on brain and skull.

o Identification of the limitations of the study to clarify the areas that could be

improved in future studies.

55

o Considering lateral impacts instead of frontal impacts on the helmet unlike

previous studies.

o Considering the most current material properties for the brain, skull and helmet.

56

Chapter 6

CONCLUSION AND FUTURE WORK

In this study the 3D models of the brain, skull and helmet were created using Solidworks

and two analyses were conducted in ANSYS to find out how the helmet will protect the

head from lateral impacts under conditions of standard tests. Normal and Von Mises

Stresses on different parts of the head and helmet were calculated and compared. The

results showed a dramatic change in the intensity of the stresses on the brain and the

skull between the first (the protected head) and the second analysis (unprotected head).

Helmet has reduced stresses significantly on brain and more than 90% of the stresses

were reduced on protected head.

Due to lack availability of high capacity CPU, the simplified versions of the models

were designed and the element numbers were reduced. The acceptable range of the

results show that the FE head model was developed properly.

It is advised that a more comprehensive model may be built that include all the layers of

the head complex to find more realistic results and since the accuracy of the model

would be higher a finer meshing will be applied to the models to increase the accuracy

of the final result in the FEA software. The proposed model may include scalp, 3-

layered skull (outer and inner tables, diploe), Dura, CSF, Pia, Falx, Tentorium, Cerebral

57

Hemispheres, Cerebellum and Brain Stem. The helmet will be improved as well to

include the outer shell and the straps.

58

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APPENDIX

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Appendix 1

! NAME! DATE! EMU!DRW.!BY! Farshad!Tavallalinia! 22.02.2013!SCALE! 1.1! DRAWING!NO.!1!

Dimensions! Centimeter!

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